How do I add a procedural confetti pattern to a plaster wall?
You will need to do this with a texture. (I have created a seamless texture for you) I don't believe there are triangle shapes with nodes, so I dont think this can be done procedural. 

Here is the Link for a seamless 4K texture I made real quick in PS. 

This one below is just a reference of what it looks like. 



Hope this helps.